Current trends toward smaller electronics products at lower costs and with greater performance capabilities are imposing greater precision requirements on the size and density of printed circuit boards. Smaller circuit boards with more densely packed circuitry requires greater positional accuracy and more accurate registration of master images on opposite sides of circuit boards. In addition, high speed production and low unit cost per circuit board are important considerations for producing low-cost products incorporating such circuit boards.
Circuit images that are produced photographically on circuit board substrates commonly originate from a master-image film that is preferably supported on, or is transferred to, a glass platen to prolong the life of the master images, and to accommodate higher-speed manual or automated production and greater precision of image registrations. Currently, glass platens 9 for supporting master-image films 12 include a peripheral surface groove 5, as illustrated in the plan view of FIG. 1, that serves as a vacuum channel 23 to assist in evacuating residual volumes of air from between the film 12 and glass platen 9, as illustrated in the cross-sectional view of FIG. 2. Glass platens 9 thus supporting master-image films 12 may be positioned in film-to-film registration on opposite surfaces of a circuit board panel 11, as illustrated in FIG. 3, within a vacuum system 23, 25, 27, 29 that promotes evacuation of residual air from between films 12, 13 and associated glass platens 9, 10 and from between films 12, 13 and circuit board panel 11. Master-image films 12, 13 thus positioned facilitate exposure of photosensitive layers on both sides of a circuit board panel 11 to sources of light in conventional manner to imprint the master images on respective sides of the panel 11.
Such conventional vacuum systems involving the associated glass platens 9, 10 for supporting the master-image films 12, 13 have several disadvantages. The peripheral groove 5 in each glass platen is commonly made narrow to inhibit pulling in and distorting of the master film 12, 13. This limits the surface area over which pressure differential can exert holding force on the film 12, 13 against the platen 9, 10 and light holding force of slippery film against slippery glass is conducive to shifting of the film on the glass with concomitant misalignments and registration problems. In addition, the dimensions of the film 12, 13 must extend critically beyond the peripheral groove in the glass platen to inhibit air leakage due to insufficient extension, and to inhibit peeling separation of the film 12, 13 from the glass platen 9, 10 due to excessive extension. Also, the resultant edges of the groove 5 at the surface of the glass platen form both undesirable and desirable seals against the film, as illustrated in FIGS. 4 and 5. Although a seal 4 formed between the film 12, 13 and outer edge of the peripheral groove 5, as illustrated in FIG. 4, is desirable to promote evacuation of residual air from between the film 12, 13 and glass platen 9, 10 over the expansive area surrounded by the peripheral groove 5, a seal 6 formed between the film 12, 13 and inner edge of the peripheral groove 5, as illustrated in FIG. 5 inhibits such evacuation and is undesirable. Also, leakage between the film 12, 13 and glass platen 9, 10 anywhere around the peripheral groove 5 is conducive to releasing the film 12, 13 for movement relative to the glass platen 9, 10. Further, vacuum seals thus formed between glass platen and master-image film are vulnerable to failure with associated loss of positional registration of the film on the platen due to variations in vacuum levels between film and platen relative to vacuum levels between platens, as later described herein, that can overcome the holding force of the vacuum between the film and platen. Multiple, spaced parallel grooves 5, as illustrated in FIG. 6, have been used to overcome some of the aforementioned disadvantages, but with only marginally improved performance and additional complexity and greater required surface areas of films 12 and platens 9.